Crosslinked polyethylene has been used as an insulating material for power cables. Such crosslinked polyethylene is formed by blending high-pressure low-density polyethylene (hereinafter referred to as LDPE) having a superior insulation property with a crosslinking agent (generally an organic peroxide) and an antioxidant, and crosslinking the molecular chains of the LDPE so that it has improved heat resistance. The crosslinked polyethylene is an excellent insulating material for alternating current (AC) transmission.
In recent years, direct current (DC) transmission, which is highly efficient for long distance lines, has been investigated; however, crosslinked polyethylene for AC cables is not resistant to polarity reversal and is thus not practical for DC transmission.
Dielectric breakdown during polarity reversal occurs because an electric field is distorted around the electrode by space charge accumulated in the insulating material and the electric field increases to twofold at polarity reversal. The electrical breakdown can be solved by providing a space charge trap site in the insulating material, as disclosed in Japanese Unexamined Patent Application Laid Open Nos. 1999-134942 and 1993-198217.
With the recent development of catalytic techniques, an ethylene-α-olefin copolymer prepared using a single-site catalyst (hereinafter referred to as sLLDPE) has been developed. As compared with an ethylene-α-olefin copolymer using a conventional Ziegler-Natta catalyst (hereinafter referred to as zLLDPE), it has become possible to obtain LLDPE having a uniform molecular weight distribution and uniform composition distribution, since the reaction site is uniform because of the use of the single-site catalyst. It is known that the highly uniform sLLDPE exhibits improved impact resistance it contains a reduced amount of low molecular weight component that decreases mechanical strength, and the number of tied-molecules in the lamella structure increases. Thus, since an improvement in impulse resistance, which is an electrical impact, can be anticipated, its use of sLLDPE as an electrical insulator has been proposed (Japanese Unexamined Patent Application Laid Open No. 1999-29616).
The combination of the above two technologies was expected to enable an insulating material to have high DC electrical properties; however, it has been revealed that this material exhibits extremely poor extrudability. That is, the uniformity of the molecular weight distribution of the sLLDPE has several problems: the resin pressure increases if extrusion is performed at the same temperature as that for LDPE; the surface of the extrusion moldings tends to have a rough skin due to low melt tension; and the processability is poor due to low melt tension (like starch syrup).
Some attempts have been done to improve the processability by blending the sLLDPE with other polyolefin resins; however, no useful technology having compatibility between DC electrical properties and processability has been developed.